The majority of U.S. adults fail to obtain adequate physical activity, despite the numerous health benefits associated with being more active. Growing interest in and evidence regarding the environmental factors that affect physical activity, such as neighborhood design and transportation infrastructure, has been hampered by cross-sectional designs that limit the ability to draw causal inference. The present study takes advantage of an upcoming marked change in the transportation environment, the introduction of light rail transit (LRT) in south King County (Seattle area) and likely changes in the neighborhood built environment around LRT stations, to better understand the causal effects of built environment on transportation-related walking and overall physical activity. Using a case-control longitudinal panel controlled design, 1000 adults living either close to (within 1 mile radius;case) or far from (control) an LRT station who are similar demographically and in initial neighborhood built environment, will be assessed prior to (baseline), soon after (post 1), and more than 2 years after (post 2) the introduction of LRT service. It is hypothesized that residents living close to LRT will have an increase in transportation-related walking (e.g., to/from LRT stations from home, work) and total physical activity relative to residents living farther away in both the short- and long-term. The short-term increases will likely be confined to LRT users, but it is hypothesized that long-term increases in transport walking and total physical activity will result from LRT use and changes in the built environment around LRT stations that promote walking. Such changes would include increases in residential density, land use mix and retail access, and pedestrian infrastructure. Individuals'transport walking will be assessed via 7-day travel diaries and physical activity assessed via accelerometry. Neighborhood built environment and changes therein will be assessed via parcel-level land use evaluation. In addition, changes in existing non-LRT transportation infrastructure (e.g., bus routes) and availability will also be considered. The prospective evaluation of changes in the same individuals'transport modes and physical activity over a period of expected marked change in transportation resources and built environment is a rare, but potent test of the impact environment can have on health behaviors. Such evaluations are needed to move forward the field of active living research and more importantly to better inform policy makers regarding consequences of changes in transportation systems and the built environment. PUBLIC HEALTH RELEVANCE: The proposed project has significance for public health policy and research, with health impact implications for the growing public transportation system in the U.S. and broader implications for the field of built environment and physical activity. This ability to better draw causal inference from the impact of built environment on transportation-related and total physical activity is critical to informing whether built environment modification can evoke such needed health behavior changes and is therefore essential to informing public health policy.